Boneland is an adult sequel to the two books for children Alan Garner published in 1960 and 1963. Colin and Susan, in The Weirdstone of Brisingamen and The Moon of Gomrath, are twins of about 12, rushing about on non-stop miraculous adventures and accepting supernatural events with almost superhuman placidity. At the end of the second book, Susan appears to be destined for an otherworldly fate or role. At the beginning of this third book, she has vanished. Her brother Colin has aged some 30 or 40 years, and their author nearly 50.

Fifty years is a long time between books, yet it seems right and natural that Garner should need five decades to gyre back round to where he began, the legend-haunted landscape of Alderley Edge, to be able to follow his story deeper into the human psyche and the dark abysm of time.

Colin has become a brilliant astrophysicist, ornithologist, and all-round savant with five or six masters' degrees, as well as being an outstanding cook, carpenter, and social misfit. He has total recall of everything that has happened to him since the age of 13, but has lost all memory of the years before that, along with his sister, and his intrepidity. The almost phlegmatically fearless child has become an anguished, supersensitive, self-absorbed man whose incoherent obsessions are driving him mad.

Colin is searching for his sister and his sanity, but the wellbeing of the world is also at stake. The descendant of men who danced and sang on Alderley Edge to keep the stars in their courses and bring the sun back from winter death, Colin is a shaman, heir of the shamans of the ice age and ages long before. He needs to find his own balance because his job is to keep the balance of the world. The rocks and caves of a reef of Triassic sandstone in Cheshire are the axis of the balance, the navel of the universe, the centre that must hold.

That centre is also in a cave in Delphi, an island in the Klamath River of California, a thousand places on Earth, and the Earth itself as revealed to the astronaut Rusty Schweickart on a walk in space – wherever human beings feel the depth of their connection to the world and take it as a sacred responsibility.

Yet this universal connection is felt as deeply local. This place is the sacred place. More mythmaker than fantasist, Garner names his chosen, actual landscape minutely, feature by feature, stone by stone, relishing the old place-names and the grand vocabulary of geology, weaving the words into a litany of confirmation, the endless repetition that keeps the end from coming, the rhythmic dance on the world's edge that maintains the world. Alderley Edge is the scene of a timeless ritual that must be re-enacted over and over by ignorant and ephemeral mortals. Personal tragedy and redemption are subsumed in the cosmic vision.

No wonder that the people of his story are less characters than masks, types, archetypes. But as imaginative literature reclaims the territories forbidden it by realism, and moves back from Elfland towards the outskirts of Manchester, it treads on risky ground. Readers looking for more than mere adventure expect characters whose behaviour and reactions are humanly comprehensible.

The child twins Colin and Susan were semi-characterless actors in a fantasy tale. Colin is both severely disturbed radio-astronomer and the man chosen from his generation to "look after the Edge" – and how to reconcile these roles in a character in a modern novel? How are the pyschic sufferings of a man so anachronistically fated and so emotionally crippled to be made comprehensible?

The author's success hinges partly on his division of the character into two – the 21st-century scientist Colin and an unnamed stone age ancestor. But in the end, his success must depend on the reader's willingness to be teased through an imaginative labyrinth by allusions, hints, puzzles, and tricks such as unascribed dialogue and undescribed location. The process of Colin's healing, the stages of therapy, the un-nesting of image within image, is fascinating, but the narration demands that the reader let the author manipulate and control, just as Colin is manipulated by the analyst.

And she, in the end, appears to have been a witch or goddess: in a serious novel, this is again risky.

Garner can count on the trust and admiration of many of his readers to see him through it, but my trust and admiration, though great, weren't always sufficient. No rereading has yet given me a clue to the meaning of the first eight lines of Boneland. Is the all-wise, wise-cracking, motorcycle-riding psychoanalyst the Witch-Crone or the Moon Mother? Is Susan one of the Pleiades? Is truth, as Garner would have it, not attainable through knowledge, only through belief?

Where all the teases and all the risks pay off, for me, is in the shadow-story of the man who "looked after the Edge" so long ago, the solitary artist-shaman of the ice age. These sections of the book are told in a charged, elliptical, symbolic, highly concrete language: "He cut the veil of the rock; the hooves clattered the bellowing waters below him in the dark. The lamp brought the moon from the blade, and the blade the bull from the rock. The ice rang."

You figure out what it's all about gradually; as with poetry, learning another language, learning to see and think differently, the demands and rewards are intense and real. It is this element of the book, in which the obsessions come into focus and a true balance is glimpsed, that will bring me back to Boneland, knowing I'll find there what no other novelist has ever given us.